Methods of making graphene oxide nanofilters

ABSTRACT

Nanofiltration of aqueous solutions or other water-based fluids in various applications, such as desalination, dialysis, seawater purification, for example, may be enhanced through precisely controlling a filtration cutoff within graphene oxide nanofilters. By initially compressing and constraining the stacked thickness of multiple graphene oxide layers deposited between porous substrates, the interlayer gap size, and thus, the filtration cutoff may be adjusted and optimized.

TECHNICAL FIELD

The disclosed technology relates generally to the field of filtrationand, more particularly, to methods of nanofiltration.

BACKGROUND

Many methods have been used for nanofiltration and desalination,including membrane filtration using polymer thin films with track-etchedpores created through bombardment or using aluminum oxide layersproduced electrochemically and etched to a desired porosity. Majordrawbacks in nanofiltration include the expense of initially creatingthe nanofilters, the limited use of the membranes and theirsusceptibility to fouling, as well as the energy (i.e., heating,cooling, and/or water pressure) requirements of the process.

Thin film composite membranes allow optimization of nanofiltrationthrough careful selection of the permeability of each individual layerthrough varying the membrane layer material. However, increased fluidpressure due to flow through the thin film composite membranes tend tocompact the varying material layers, thus altering their carefullychosen porosity, undermining the molecular size cutoff for thefiltration module.

Filtering aqueous solutions and other water-based fluids for purposes,such as dialysis and seawater purification, for example, could begreatly enhanced if a precise filtration cutoff could be easily tuned.Many filters and other separation devices have an allowable threshold(i.e., filtration cutoff) for the size of molecules able to pass throughthe filter without being caught or otherwise stopped. This threshold canhave a wide range depending on the filter type.

On the nanoscale level, most conventional filtration is limited tomembrane filtration, which uses particularly sized pores to limit themolecules passing through the membrane. The specific filtrationapplication both guides and limits the selection and design of theporosity and filtration cutoff for the membranes. The filtration cutofffor membrane filters may only be as precise as the pore creation processallows. For example, membranes created through interfacialpolymerization are inexpensive, but have a wider than desirable poresize distribution. Then, once the pores are created in the membrane,their size is fixed and cannot easily be adjusted. Further, themembranes themselves may be susceptible to leaks or tears over time,which destroys the initial filtration cutoff.

Therefore, in order to provide greater precision and flexibility innanofiltration and desalination systems, new membrane designs areneeded, which include sharper filtration cutoffs and modularity formultiple applications. Research on the permeability of graphene oxidefilms has shown that graphene oxide laminates do not readily permeateions and molecules with hydrated radii beyond an acceptable range.

SUMMARY

According to aspects of the present disclosure, methods formanufacturing nanofiltration systems include deposition of grapheneoxide layers on a first porous surface and compression of those layersbetween the first and a second porous surface up to the yield point ofthe graphene oxide layers. The nanofiltration system may includedetermining the effective number of graphene oxide layers between theporous surfaces, as well as the effective interlayer gap size. Usingpressure or distance adjusting controls between the porous surfaces, theeffective interlayer gap size between the graphene oxide layers of thenanofiltration system may be set and varied. Further, severalnanofiltration membranes may be made simultaneously through dividing theporous surfaces and deposition layers into smaller sections afterstacking.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of an example process of makingand using a graphene oxide nanofilter, in accordance with certainembodiments of the disclosed technology.

FIG. 2 is a cross-sectional side view of an example graphene oxidenanofilter with an adjusting control for setting an interlayer gap size,in accordance with certain embodiments of the disclosed technology.

FIG. 3 is a cross-sectional side view of an example nanofiltrationsystem using multiple graphene oxide nanofilters with adjustablefiltration cutoffs, in accordance with certain embodiments of thedisclosed technology.

FIG. 4 is a cross-sectional side view of an example nanofiltrationsystem using tangential flow, in accordance with certain embodiments ofthe disclosed technology.

FIG. 5 is a cross-sectional side view of an example nanofiltrationsystem using multiple graphene oxide nanofilters and tangential flowpaths, in accordance with certain embodiments of the disclosedtechnology.

FIG. 6 is a cross-sectional side view of an example nanofiltrationsystem using tangential flow and recirculation, in accordance withcertain embodiments of the disclosed technology.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes structures and methods for setting andadjusting filtration cutoffs for the maximum molecule size allowed topermeate within filters made in bulk using deposited graphene oxidelayers. The initial setup of these nanofilters includes compressing andconstraining the graphene oxide layers to control their swelling inwater, thereby governing the effective channel size of the nanofilter.The flexibility of the filtration cutoff within the graphene oxidefilters enables arbitrary filtration processes, which would otherwiseeach require their own specially made filter or membrane with a fixedporosity having only one specific filtration cutoff range that morerapidly degrades due to fouling or enlarging, eroding wear on the pores.

In addition to sharp filtration cutoff limits, graphene oxide laminateshave exhibited unique capillary pressure characteristics for aqueoussolutions and other water-based fluids. Without being limited to theory,it is believed that stacked layers of graphene oxide form pristineregions of capillaries, which allow near frictionless flow of waterthrough the graphene oxide layers. Ions present in the water are drawninto the graphene oxide capillaries due to a force gradient caused bythe energy gain in interactions between the ions and the graphene walls.The graphene oxide capillaries rapidly transport molecules withmolecular or hydrated radii that can fit within the capillaries' width.The hydrated radius for an ion is the effective radius of the ion andits waters of hydration. Thus, the size of the capillaries within thegraphene oxide layers controls the size of the molecules able topermeate the stacked layers (i.e., a filtration cutoff).

As illustrated in FIG. 1, the effective cross-section or width of thegraphene oxide capillaries is correlated to an interlayer gap size 108between the stacked graphene oxide layers 102. Mechanically or otherwiseconstraining the swelling of the stack of graphene oxide layers 102 whenimmersed in water may control the interlayer gap size 108. Thus, theconstraint placed on the stack thickness of the deposited graphene oxidelayers 102 directly controls the interlayer gap size 108 and thereby thefiltration cutoff for molecules able to rapidly pass through thegraphene oxide capillaries.

FIG. 1 illustrates example assembly steps for making nanofilters inaccordance with some embodiments of the disclosure. First, grapheneoxide layers 102 with a layer thickness t are deposited or stacked ontoa porous substrate 104. As a non-limiting example, the graphene oxidelayers 102 may be deposited to a desired stack thickness of betweenabout 0.1 μm and about 10 μm. The graphene oxide layers may be formed invarious ways, such as by using graphene oxide flakes with a lateralextent from about 1 μm to about 100 μm or graphene oxide paper, forexample. In some embodiments, the graphene oxide layers are formedthrough deposition using vacuum filtration and/or precipitation. Inother embodiments, the graphene oxide layers may be formed usinggraphene oxide synthesis methods, such as the Tang-Lau method, forexample. Additionally, various components and/or materials may providethe flat surface, rigidity, and controlled pore size of the poroussubstrate 104, acting as a coarse filter relative to the channel size ofthe graphene oxide layers, yet with small pores relative to the diameterof graphene oxide flakes, for example. In some embodiments, the poroussubstrate may be a filter sold under the trademark ANOPORE by WhatmanInternational Ltd., or another anode etched aluminum oxide with a smoothsurface or film, having an effective pore size of about 10 nm to about100 nm. Alternatively, the porous substrate may have a pore size ofabout 2 μm or less, as discussed below in relation to FIG. 2.

Next, another porous substrate 104 may be placed atop the graphene oxidelayers 102. In some embodiments the top porous substrate may be a filtersold under the trademark ANOPORE by Whatman International Ltd., or othersmooth surface with smaller pore sizes, while the bottom poroussubstrate is rougher with larger pore sizes. The two porous substrates104 may sandwich the graphene oxide layers 102 as the graphene oxidelayers 102 are compressed to a minimum thickness d₀, such that aseparation distance 106 between the top and bottom porous substrates 104(i.e., the stack thickness of the graphene oxide layers 102) is reducedto a width within the elastic regime of the material's stress-straincurve before the yield point, approaching incompressibility.

Once reached, the minimum thickness d₀ of the graphene oxide layers 102may be measured using techniques, such as interferometry or sensingtranslational and/or rotational positions of fine pitch screws, inchworm piezo actuators, and/or springs, for example. Measuring the minimumthickness d₀ may enable the calculation of the effective number ofgraphene oxide layers 102 based on the known layer thickness t, usingthe formula:d ₀ =ntwhere n is the number of graphene oxide layers and t is the knownthickness of one graphene oxide layer. The number of graphene oxidelayers n may allow for the interpolation of the interlayer gap size 108based on the separation distance 106 set and/or adjusted once thegraphene oxide layers 102 are immersed in water, using the formula:D˜n(t+τ)=d ₀ +nτ=d ₀(1+τ/t)where D is the set separation distance 106 and τ is the effectiveinterlayer gap size 108. Without being bound to theory, it is believedthat the effective spacing τ may approximate the actual interlayer gapsize because the acting intercalation forces tend to equalize theinterlayer gap spacings between the graphene oxide layers.

There are many ways to mechanically constrain the deposited grapheneoxide layers 102 during the initial compression to the minimum thicknessd₀ of the graphene oxide layers 102 and/or during adjustment to theseparation distance 106 while the graphene oxide layers 102 are immersedin water. In some embodiments, the graphene oxide layers 102 may becompressed using an adjusting control 110 controlling a vice or clampingaction on support rods inserted into the porous substrates 104, as shownin FIG. 1. In other embodiments, graphene oxide layers may be compressedbetween porous support plates, as discussed below and shown in FIG. 2.The separation distance 106 between the porous substrates 104 isproportional to the amount of space the graphene oxide layers 102 arepermitted to occupy, and thus, their interlayer gap size 108.

The separation distance 106 between the porous substrates 104 may beprecisely set or adjusted using the adjusting control 110 in a varietyof ways. In some embodiments, fine pitch screws directly or indirectlyconnect the porous substrates and may precisely control their relativelateral movement. In other embodiments, inch worm piezo actuation maycontrol the separation distance between the porous substrates. In yetother embodiments, spring mounts and/or pneumatic action may control thepressure applied to the porous substrates and thereby regulate theseparation distance. Although not illustrated, there may be additionalsensors used with the nanofilter to measure the actual separationdistance. This separation distance data may be used by a controller toaccount for hysteresis, deformation, and/or wear in the nanofilterand/or mechanical constraint device used to set the separation distance.

As illustrated in FIG. 1, the set interlayer gap size 108 may controlthe filtration cutoff in the nanofiltration module or nanofilter 100 forregulating which molecules may permeate through to the bottom poroussubstrate 104. There are varying ways to determine which separationdistance 106 should be used to set the desired interlayer gap size 108of the nanofilter 100 alternatively or in addition to calculating thenumber of graphene oxide layers described above. Due to the varyingdegrees and types of structural changes to graphite during synthesis ofgraphite oxide, the filtration cutoff response to changes in theseparation distance may fluctuate depending on the particular synthesismethod and/or deposition method chosen. For this reason, it may beadvantageous to test a nanofilter across a range of separation distancesto empirically determine the correlation between the separationdistance, interlayer gap size, and/or filtration cutoff. Duringnanofilter testing, the observed filtration cutoffs may be mapped tospecific separation distances or ranges of separation distances. Thesesets of separation distances may be programmed into a controller and/orassociated memory for easily recalling different filtration cutoffstates within the nanofilter or nanofiltration module.

FIG. 2 illustrates another example nanofiltration module or nanofilter200 for adjustably setting the filtration cutoff in varying filtrationsystems and/or applications. The nanofilter 200 includes an adjustingcontrol 210 for setting the separation distance between two poroussupport plates 206. The porous support plates 206 may have a roughersurface and larger pore size, as discussed above. Sealed with an o-ring208, the porous support plates 206 may surround secondary filters 204,which sandwich the central graphene oxide layers 202. Although shown asan o-ring, the o-ring 208 may be any type of seal or sealing materialfor laterally sealing the adjustable interlayer gap zone that includesthe graphene oxide layers. The secondary filters 204 may be filters soldunder the trademark ANOPORE by Whatman International Ltd., and/or othersmooth surfaced media with a smaller pore size, as described above. Thesecondary filters 204 may provide a preliminary filtration cutoff, whilethe porous support plates 206 provide both structural strength andpermeability. Using combined layers of porous support plates andsecondary filters with a smaller pore size may advantageously reducematerials costs by minimizing the width of the more expensive secondaryfilters without undermining the compressive strength used forconstraining and setting the stacked thickness of the graphene oxidelayers.

FIG. 3 illustrates a nanofiltration module or assembly 300 that includesmultiple adjustable graphene oxide nanofilters for various filtrationsystems and/or applications. A first nanofilter 220 may be arrangedupstream of a second nanofilter 230 in the nanofiltration module 300.This serial arrangement of the first nanofilter 220 and secondnanofilter 230 may enable modular multilayer filtration and/orsimultaneous separation of multiple molecular sizes. The adjustedseparation distance within the first nanofilter 220 may be larger thanthe adjusted separation distance in the second nanofilter 230. At anentry stage 302 in the nanofiltration module 300, an entry valve 312 maycontrol the flow of a feed fluid that includes smaller-sized molecules322 and medium-sized molecules 324 as well as larger-sized molecules(not shown). A first stage 304 of the nanofiltration module 300 includesan outlet valve 314 for controlling the flow of larger, medium, andsmaller molecules from the nanofiltration module 300 as well asregulating the pressure of the feed fluid in the first stage 304. Thefirst nanofilter 220 separates the first stage 304 from a second stage306 in the nanofiltration module 300 and prevents the larger moleculesfrom entering the second stage 306, while allowing permeation of thesmaller-sized molecules 322 and the medium-sized molecules 324. Thesecond stage 306 may include an outlet valve 316 for controlling theflow of smaller-sized molecules 322 and medium-sized molecules 324 fromthe nanofiltration module 300. The second nanofilter 230 separates thesecond stage 306 from a third stage 308 in the nanofiltration module 300and prevents the medium-sized molecules 324 from entering the thirdstage 308, while allowing permeation of the smaller-sized molecules 322.The third stage 308 may include an outlet valve 318 for controlling theflow of smaller-sized molecules 322 from the nanofiltration module 300.Further, the nanofiltration module 300 may include an exit valve 320 atan exit stage 310 of the nanofiltration module 300 for controlling theflow of smaller-sized molecules 322 from the nanofiltration module 300as well as regulating the pressure of the feed fluid and filtrates inthe entire nanofiltration module 300.

FIG. 4 illustrates a nanofiltration system 400 with tangential flowfiltration across an adjustable graphene oxide nanofilter 402. Withinthe tangential flow arrangement of the nanofiltration system 400, a feedfluid 420, including larger-sized molecules and medium-sized molecules,flows into the nanofiltration system 400. The separation distance of theadjustable graphene oxide nanofilter 402 may be set such that thefiltration cutoff is lower than the effective hydrated radius of thelarger-sized molecules. Thus, due to permeation of the medium-sizedmolecules through the adjustable graphene oxide nanofilter 402, anexiting fluid 422 may have a lower concentration of medium-sizedmolecules compared to the entering feed fluid 420, and a filtrate fluid424 may include medium-sized molecules without any larger-sizedmolecules. The concentration of the filtrate fluid 424 may be adjustedby altering the concentration and/or pressure of the feed fluid 420.

FIG. 5 illustrates a nanofiltration system 500 with tangential flowfiltration across multiple adjustable graphene oxide nanofilters 502 and504. Within the tangential flow arrangement of the nanofiltration system500, a feed fluid 520, including larger-sized molecules, medium-sizedmolecules, and smaller-sized molecules, flows into the nanofiltrationsystem 500. The separation distance of the adjustable graphene oxidenanofilter 502 may be set such that the filtration cutoff is lower thanthe effective hydrated radius of the larger-sized molecules. Thus, dueto permeation of the medium-sized molecules and the smaller-sizedmolecules through the adjustable graphene oxide nanofilter 502, anexiting fluid 522 may have a lower concentration of both medium-sizedmolecules and smaller-sized molecules compared to the entering feedfluid 520, and a filtrate fluid 524 may include medium-sized moleculesand smaller-sized molecules without any larger-sized molecules. Further,the separation distance of the adjustable graphene oxide nanofilter 504may be set such that the filtration cutoff is lower than the effectivehydrated radius of the medium-sized molecules. Thus, due to permeationof the smaller-sized molecules through the adjustable graphene oxidenanofilter 504, a filtrate fluid 526 may include smaller-sized moleculeswithout any medium-sized molecules or larger-sized molecules. Theconcentrations of the filtrate fluids 524 and 526 may be adjusted byaltering the concentration and/or pressure of the feed fluid 520.

FIG. 6 illustrates a nanofiltration system 600 similar to thenanofiltration system 500 of FIG. 5, but further including recirculationloops. Within the tangential flow arrangement of the nanofiltrationsystem 600, a feed fluid 620, including larger-sized molecules,medium-sized molecules, and smaller-sized molecules, flows into thenanofiltration system 600. The separation distance of an adjustablegraphene oxide nanofilter 602 may be set such that the filtration cutoffis lower than the effective hydrated radius of the larger-sizedmolecules. Thus, due to permeation of the medium-sized molecules and thesmaller-sized molecules through the adjustable graphene oxide nanofilter602, an exiting fluid 622 may have a lower concentration of bothmedium-sized molecules and smaller-sized molecules compared to theentering feed fluid 620, and a filtrate fluid 626 may includemedium-sized molecules and smaller-sized molecules without anylarger-sized molecules. Further, the exiting fluid 622 is recirculatedand input back into the nanofiltration system 600 with the feed fluid620. The flow rate of the recirculation loop of the exiting fluid 622may be set and/or adjusted based on a desired constant concentrationlevel and/or flow rate of the mixed fluids entering the nanofiltrationsystem 600 in a continuous operation mode. Alternatively, thenanofiltration system 600 may be operated in a batch mode where, afteran initial introduction of a fixed volume of the feed fluid 629, theexiting fluid 622 is recirculated through the nanofiltration system 600until a desired concentration and/or volume of the filtrate fluid 626 isreached. Moreover, the separation distance of the adjustable grapheneoxide nanofilter 604 may be set such that the filtration cutoff is lowerthan the effective hydrated radius of the medium-sized molecules. Thus,due to permeation of the smaller-sized molecules through the adjustablegraphene oxide nanofilter 604, a filtrate fluid 628 may includesmaller-sized molecules without any medium-sized molecules orlarger-sized molecules. Similar to the recirculation loop of the exitingfluid 622, the filtrate fluid 626 may be recirculated into thenanofiltration system 600 between the adjustable graphene oxidenanofilters 602 and 604. The recirculation loop for the filtrate fluid626 may be continuously run at a set flow rate correlated with thecontinuous-mode flow operation of the feed fluid 620, or may berecirculated until a desired concentration and/or volume of the filtratefluid 628 is reached in the batch-mode operation.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A method comprising: depositing graphene oxidelayers on a first porous surface; adding a second porous surface atopthe graphene oxide; applying pressure to the first and second poroussurfaces up to a yield point of the graphene oxide layers beyond whichthe layers become incompressible; correlating the distance between thefirst and second porous surfaces to an interlayer gap size between thegraphene oxide layers; and determining a filtration cutoff for a filterusing the porous surfaces based upon the interlayer gap size.
 2. Themethod of claim 1, wherein depositing graphene oxide layers includesdepositing graphene oxide flakes.
 3. The method of claim 1, whereindepositing graphene oxide layers is accomplished using vacuumfiltration.
 4. The method of claim 1, further comprising: determininghow many layers of graphene oxide are deposited.
 5. The method of claim4, wherein determining how many layers of graphene oxide are depositedincludes determining a compressed thickness of the graphene oxidelayers.
 6. The method of claim 1, further comprising: adding a firstsupport plate below the first porous surface; and adding a secondsupport plate above the second porous surface.
 7. The method of claim 6,wherein applying pressure to the first and second porous surfacesincludes decreasing a distance between the first and second supportplates.
 8. The method of claim 1, wherein applying pressure to the firstand second porous surfaces includes decreasing a distance between thefirst and second porous surfaces.
 9. The method of claim 1, furthercomprising setting a predetermined distance between the first and secondporous surfaces prior to applying pressure.
 10. The method of claim 1,further comprising: dividing the first and second porous surfaces anddeposited graphene oxide layers into two or more stacks, wherein eachstack includes the first and second porous surfaces and depositedgraphene oxide layers.
 11. A method comprising: depositing layers ofgraphene oxide between a first and second support plate to form a filtermodule; compressing the first and second support plates; determining thethickness of the compressed graphene oxide layers; immersing the filtermodule in water; determining a separation distance based on at least oneof a desired filtration cutoff and a desired interlayer gap size; andseparating the first and second support plates by the separationdistance.
 12. The method of claim 11, wherein determining the thicknessof the compressed graphene oxide layers includes determining how manygraphene oxide layers have been deposited.
 13. The method of claim 11,wherein determining a separation distance includes: separating the firstand second support plates by a set of test distances, determining atleast one of an actual filtration cutoff and an actual interlayer gapsize, resulting from the test distance, and mapping each test distanceagainst at least one of each determined actual filtration cutoff andactual interlayer gap size.
 14. The method of claim 11, furthercomprising: determining the separation distance based on at least one ofthe determined graphene oxide layers' thickness, an actual interlayergap size, and a desired filtration cutoff.
 15. The method of claim 11,wherein separating the first and second support plates is achievedthrough adjusting a fine pitched screw.
 16. The method of claim 11,wherein separating the first and second support plates is achievedthrough actuating an inch worm piezo.
 17. The method of claim 11,wherein separating the first and second support plated is achievedpneumatically.
 18. The method of claim 11, further comprising: insertinga first porous substrate filter between the graphene oxide layers andfirst support plate; and inserting a second porous substrate filterbetween the graphene oxide layers and second support plate.
 19. Themethod of claim 11, further comprising: adjusting the separationdistance between the first and second support plates after separatingthe first and second support plates initially.